The present invention relates in general to electrically conducting components and more particularly to the type of components which have projecting terminals extending from an encapsulating body and which may be engaged with female pressure clip connectors or plugged into female connectors which are commonly provided in elongate blocks of insulating material. The preferred embodiment of the present invention relates more specifically to a miniature size fuse for use in protecting electrical circuits, and especially for use in protecting electrically operated components in automotive vehicles.
The inventor of the present invention has developed an encapsulated type fuse which is disclosed in U.S. Pat. No. 3,914,863 and U.S. Pat. No. 3,832,664. The fuse disclosed in those patents does not have terminals projecting from the fuse. Further, the fuse disclosed in those patents has a cylindrical shape with cylindrical end caps in electrical contact with the fuse wire which is coiled or bent on each end of the fuse. The end caps are held against the bent ends of the fuse wire at each end of the fuse body and the end caps are mechanically secured to the fuse body.
The above-described fuse, which functions well for its intended purpose, cannot be used in plug-in type fuse receiving blocks and requires a number of manufacturing steps to bend the fuse wire ends and apply the end caps. It would be desirable to provide a relatively simple encapsulated fuse which does not require the additional steps of bending of the fuse wire ends and the securing of end caps thereto. Further, it would be desirable to provide a fuse with projecting terminals having a graspable structure which would be an aid in inserting or removing of the fuse. Yet such a structure should have a relatively smooth surface and preferably not be susceptible to accidental "snagging" which might dislodge the fuse.
Plug-in fuses have been developed today for use in automotive and other circuits and have a compact, substantially rectangular or square shape about 3/4 inch on each side with a width of under 1/4 inch. Such fuses have a fusible link surrounded by an insulating housing and have terminal-forming blade portions projecting from the housing. The blades and fusible link are made of metal and may be three separate pieces connected together or may be a single stamped piece of metal. For purposes of further discussion the term "fuse element" will be used to means an electrically conducting structure (commonly metal) the major portion of which is contained within the insulating housing or fuse body. The fuse element typically consists of (1) two conductor terminals (such as blades, posts, or other structures which commonly extend from the housing) and (2) one fusible link connecting the two terminals inside the housing.
A fuse having a fuse element consisting of a single piece stamping of a strip of fuse metal enclosed in a plastic housing is disclosed in the U.S. Pat. No. 3,909,767 to Williamson et al. A three-piece fuse element in which a fusible link is soldered to two blades within a plastic housing is disclosed in the U.S. Pat. No. 3,775,724 to Mamrick et al.
Fuses of the above-described types have met with significant commercial success and in general, have functioned satisfactorily for the intended purpose. However, in spite of their apparent simplicity, fuses of the above-described types have a number of drawbacks.
One disadvantage of the above-described types of fuses is that the insulating enclosures or housings surrounding the fuse element (blades and connecting fusible link) are not in intimate contact with the fusible link and the junctions with the terminal blades. The housing may or may not be open at the end from which the terminal-forming blade ends protrude. In either case, the housing is spaced away from the fusible link and/or upper portions of the blades to which the link is connected. The insulating housing thus defines a volume or space around the fusible link and necessarily contains an atmosphere of some kind. Generally, the housing interior communicates with the ambient atmosphere outside of the housing. Even those housings which totally enclose the upper portions of the terminal-forming blades and the fusible link are commonly not leak-tight and do allow air to pass into the housings to help dissipate the heat generated by the fuse element. Thus, it is possible for foreign matter, especially dust, airborne corrosive particles, and other such airborne contaminants, to enter the insulating housing and be deposited on the fusible element. To the extent that such airborne contaminants may react with, corrode, or otherwise have a deleterious effect upon the fusible link and/or upper portion of the terminal-forming blades, the admission of such contaminants is undesirable.
Even where an insulating housing could be made completely airtight, extraordinary care would have to be taken during manufacture to carefully control the atmosphere within the sealed insulating housing. If such care were not taken during manufacture, contaminants could be sealed within the housing during manufacture. Presumably, this problem would be somewhat alleviated by forming the insulating housing around the fusible link in a vacuum or in some inert atmosphere. Obviously, such a structure would be more complicated and expensive to manufacture than a structure not manufactured in a vacuum or in an inert atmosphere.
For those types of fuses in use today in which the housing is open to atmosphere or is not airtight, the internal volume within the housing around the fusible link is thus going to contain an atmosphere which may then vary in pressure as well as composition (e.g., water vapor content and contaminants) according to the variations in the ambient atmosphere. Thus, thermal conductivity and heat capacity of the atmosphere within the housing will vary to the extent of their dependence upon the above-listed parameters. Since the atmosphere within the housing functions, to some extent, to dissipate heat generated by the fuse element, variations in the atmosphere could cause slight variations in the rate of dissipation of heat generation from the fuse element. In some cases, depending upon the size of the fuse, the type of fusible link material employed, the current rating of the fuse and the normal design current passing through the fuse, the actual current level at which the fuse may "blow" could vary from the intended design rating.
With those types of fuses in use today in which the housing is open to the atmosphere or is not air-tight, there is the possibility that when the fuse blows, the arcing of the vaporizing fusible link can, because of the communication with the ambient atmosphere, ignite or explode combustible gases that may be present. This is especially important with respect to fuse applications in automotive vehicles, airplanes, ships and the like wherein during and/or after an accident or collison, one or more fuses may blow and where gasoline vapors or other fuel fumes may be present. Thus, it would be desirable to provide a totally encapsulated fuse wherein the arcing of the fusible link is entirely contained or submerged within a noncombustible material to prevent its communication with ambient atmosphere.
In a fuse having a fuse element consisting of a single piece stamping of fuse metal enclosed in a plastic housing, such as disclosed in the above-mentioned U.S. Pat. No. 3,909,767, the portion of the inner surface of the housing surrounding the fuse link is spaced away from the fuse link. In fuses of this type, the fuse element is typically stamped from zinc which has a relatively high melting point. If the fusible link portion of the fuse element is not spaced away from the interior surface of the plastic housing, the housing may melt when the fuse blows or in overload modes prior to blowing.
With those types of plug-in fuses having a fuse element stamped from a single piece of metal, there are other disadvantages. For example, the terminal-forming blade portions, as well as the fusible link, are necessarily formed from the same sheet of fusible metal. This presents certain problems. If it is desired to have a fusible link comprised of a very soft and low-melting alloy, the terminal-forming blades will be undesirably soft and ductile. Further, to the extent that a fusible link material is desired with a composition that is very carefully controlled and that is made from an expensive metal or metal alloy, the cost of the fuse is unnecessarily increased because the terminal-forming blade portions must also necessarily be composed of the more expensive material.
Typically, those types of plug-in fuses having fuse elements stamped from a single piece of metal use a zinc alloy for the fuse element. With such a one-piece element, both the fusible link and the terminal posts or terminal-forming blade portions of the fuse element are comprised of the zinc alloy. This has a serious drawback, however, because the plug-in fuse receiving housing commonly incorporates female pressure clip connectors which are made from brass. Owing to the galvanic cell created between the zinc alloy terminal-forming blades and the brass female pressure clip connectors, electrolytic corrosion can be a problem. Thus, it would be advantageous to provide a plug-in fuse wherein the terminal-forming blades are made of brass instead of zinc alloy. However, plug-in fuses having a fuse element stamped from a single piece of material are not easily made from brass owing to problems that brass presents in the stamping process. Fusible links typically have a relatively small cross section with a thickness measured in thousandths of an inch. It is very difficult to stamp the fusible link from brass to the proper dimensions within the necessary tolerances. Other metals, such as some zinc alloys, are much more suitable for the stamping process and can be stamped with a relatively small cross section within the necessary tolerances. Consequently, in order to take advantage of the strength and durability characteristics of brass, and in order to avoid electrolytic corrosion problems, it would be desirable to provide a plug-in fuse having brass blades but having a fusible link which can be formed from a different metal that is well-suited for the forming processes employed.
Further, it would be beneficial to provide a low melting point alloy fusible links, which has a lower melting point than zinc, or brass, to preclude localized melting and/or deformation of the plastic housing.
There is another disadvantage with plug-in fuses having a fuse element stamped from a single piece of metal. As each fuse element is stamped, the relatively complexly shaped die wears and the die cavity becomes larger. Therefore, as many fuse elements are stamped, the size of the fuse elements becomes larger and may eventually exceed the design tolerances. Though a die can be replaced when it has worn enough to produce stampings beyond the design tolerance, replacment of a complete die is expensive and is preferably avoided whenever possible. In contrast, in a fuse element having a separate, wire fusible link, the wire can be drawn through a multiple of relatively simple wire drawing dies of decreasing diameter wherein the very last (downstream) die is the smallest and effects the final wire diameter. Consequently, when the last die wears beyond the design tolerance, only this last die need be replaced. Replacement of just the last die is relatively inexpensive.
In a fuse that has a fusible link or wire surrounded by a housing or other material, it is important to provide a means whereby the heat generated in the fusible link can be uniformly conducted from all portions of the link so that the link will vaporize, as desired, in the "middle" of the fuse body and at the design current rating.